A major cause of technical difficulties and limitations in the operation of metal vapour lasers is the method by which the metal atoms are established within the laser discharge volume. Conventional methods of vaporizing the metal to provide metal atoms have limitations.
Direct vaporization of the metal (Walter et al 1966) using either an external oven or discharge heating requires high temperatures, about 1500.degree. C. for copper, to be achieved. High power copper and gold lasers exploit the discharge energy to heat a tube. The need for ovens also makes it difficult to construct the fast discharge circuitry needed for excitation of other potentially interesting self-terminating neutral metal laser transitions.
The use of a volatile compound of the metal (Chen et al 1973), for example a metal halide, reduces the temperatures needed to attain an adequate metal concentration. However, relatively few metals possess suitable compounds. The difficulty in constructing closely coupled discharge circuitry needed for fast discharges in self-terminating lasers, however, still remains. Associated with the use of volatile metal compounds is the need to dissociate the molecule. In the case of neutral transition lasers this requires the use of double or continuously pulsed discharges, with all the limitations that the constraints set by the dissociation-reassociation cycle impose upon the system. In particular, the dissociation process results in an undesirable population of the lower laser level. In contrast, continuous-wave (cw) metal ion lasers use a dc discharge which produces both dissociation and excitation. The dissociation is virtually complete so that the vapour pressure is controlled by tube temperature and consequently the excitation may be separately optimized (Brandt and Piper 1977).
Another commonly used method of metal production is by sputtering the metal atoms from a cathode of the desired material (Gersternberger et al 1980). To date, the metal density required to support laser action has been produced by cathodic sputtering using a hollow cathode discharge. Use of a hollow cathode also greatly increases the extent of ionization in the buffer gas over that found in the more common positive column type gas discharge. This combined with enhancement of charge transfer reactions of the type: EQU B.sup.+ +M.fwdarw.B+(M.sup.+)*+.DELTA.E
where B, B.sup.+ and M represent buffer gas atoms, ions and metal atoms respectively and (M.sup.+)* represents metal ions in excited states, results in the hollow cathode discharge being particularly suitable for charge transfer metal ion lasers. Indeed, thus far, sputtering has been restricted to cw metal ions lasers, which require relatively high discharge currents (1 to 100 A).
A theory for the operation of this type of hollow cathode ion laser has been developed by Warner et al (1979). In their model the equations for the densities of the rare gas ions, metal vapour atoms and ions are coupled via the charge transfer reaction as above. As a consequence of this the discharge current cannot be independently varied or optimized with respect to the desired metal atom density. This restriction is the major practical disadvantage of sputtering lasers to date.
Only sputtering is applicable to a wide range of metals and can produce a concentration of metal atoms of the order 10.sup.14 cm.sup.-3 continuously at room temperature.
In existing sputtering lasers, both the sputtering of the metal and the excitation of the lasing transition are accomplished by a single discharge in a static noble gas metal vapour mixture. The conventional methods do not allow separate control and optimisation of the metal vapour concentration and the excitation discharge. The object of the present invention is to provide a metal vapour laser for operation at room temperature which overcomes some or all of the abovementioned drawbacks.